Recently, portable electric and electronic devices such as notebook computers, mobile phones, PDAs, and the like have entered widespread in daily life and thus rechargeable secondary batteries, which enable such portable electric/electronic devices to operate even when a separate power supply is not available, are used. Among these secondary batteries, lithium secondary batteries developed in the early 1990's are mainly used because they have higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni—Cd, sulfuric acid-lead batteries, and the like. Such lithium secondary batteries tend to be developed as high-capacity lithium secondary batteries according to demand of consumers who prefer lighter and more compact products.
A lithium secondary battery includes a cathode including, as a cathode active material, a lithium-containing composite oxide such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, or the like.
Meanwhile, an anode of a lithium secondary battery includes various carbon materials as anode active materials. A carbon material used as an anode active material is classified as a crystalline graphite material and an amorphous carbon material. In general, the crystalline graphite material may be artificial graphite, natural graphite, Kish graphite, or the like. In addition, the amorphous carbon material may be soft carbon obtained by calcining coal-based pitch or petroleum-based pitch at high temperature, hard carbon obtained by calcination of a polymer resin such as phenolic resin, or the like.
In general, a crystalline graphite material and an amorphous carbon material have advantages and disadvantages in terms of voltage smoothness, charge and discharge efficiency, and reactivity with an electrolyte and thus, to manufacture high-capacity and high-efficiency batteries, the two materials are used in combination by a method such as coating or the like, thereby enhancing battery performance.
Recently, crystalline graphite is mainly used by surface coating treatment, and coating of natural anode materials is mainly performed by aqueous anode coating. In the past, a non-aqueous anode coating method was used and, here, N-methyl-2-pyrrolidone (NMP) and PVdF were used as a solvent and a binder, respectively. Currently, water and styrene butadiene rubber (SBR) are widely used as a solvent and a binder, respectively, based on an aqueous anode coating method. That is, an anode is fabricated through coating of natural graphite having high specific surface area as an anode active material by using a smaller amount of SBR than that of existing PVdF, whereby an absolute amount of the anode active material in the anode is increased, which enables production of high capacity batteries.
When natural graphite having high specific surface area is used as an anode active material, however, problems such as clogging of a filter or reduction in slurry dispersibility in a mixing process may occur in electrode fabrication.
Thus, use of anode active materials including graphite obtained by coating natural graphite with amorphous carbon and calcining the resulting material and sheet-type graphite is also proposed.
These technologies require complicated processes such as coating of natural graphite with pitch, calcination thereof, or the like and thus are undesirable in terms of overall cost and manufacturing processability of batteries.
Therefore, there is a high need to develop technologies that fundamentally address these problems and to develop anode materials that are not coated with pitch, enhance battery cycle characteristics, and are desirable in terms of both cost and manufacturing processability.